1. Field
The present embodiment relates to vehicle wheels, and particularly to shields and devices used to reduce drag on rotating vehicle wheels.
2. Description of Prior Art
Inherently characteristic of rotating vehicle wheels, and particularly of spoked wheels, aerodynamic resistance, or parasitic drag, is an unwanted source of energy loss in propelling a vehicle. Parasitic drag on a wheel includes viscous drag components of form (or pressure) drag and frictional drag. Form drag on a wheel generally arises from the circular profile of a wheel moving though air at the velocity of the vehicle. The displacement of air around a moving object creates a difference in pressure between the forward and trailing surfaces, resulting in a drag force that is highly dependent on the relative wind speed acting thereon. Streamlining the wheel surfaces can reduce the pressure differential, reducing form drag.
Frictional drag forces also depend on the speed of wind impinging exposed surfaces, and arise from the contact of air moving over surfaces. Both of these types of drag forces arise generally in proportion to the square of the relative wind speed, per the drag equation. Streamlined design profiles are generally employed to reduce both of these components of drag force.
The unique geometry of a wheel used on a vehicle includes motion both in translation and in rotation; the entire circular outline of the wheel translates at the vehicle speed, and the wheel rotates about the axle at a rate consistent with the vehicle speed. Form drag forces arising from the moving outline are apparent, as the translational motion of the wheel rim must displace air immediately in front of the wheel (and replace air immediately behind it). These form drag forces arising across the entire vertical profile of the wheel are therefore generally related to the velocity of the vehicle.
As the forward profile of a wheel facing the direction of vehicle motion is generally symmetric in shape, and as the circular outline of a wheel rim moves forward at the speed of the vehicle, these form drag forces are often considered uniformly distributed across the entire forward facing profile of a moving wheel (although streamlined cycle rims can affect this distribution somewhat). This uniform distribution of pressure force is generally considered centered on the forward vertical wheel profile, and thereby in direct opposition to the propulsive force applied at the axle, as illustrated in FIG. 12.
However, as will be shown, frictional drag forces are not uniformly distributed with elevation on the wheel, as they are not uniformly related to the speed of the moving outline of the wheel rim. Instead, frictional drag forces on the wheel surfaces are highly variable and depend on their elevation above the ground. Frictional drag must be considered separate from form drag forces, and can be more significant sources of overall drag on the wheel and, as will be shown, thereby on the vehicle.
The motion of wheel spokes through air creates considerable drag, especially at higher relative wind speeds. This energy loss is particularly critical in both bicycle locomotion and in high-speed vehicle locomotion. Previous efforts to reduce this energy loss in bicycle wheels have included bladed-spoke designs; the addition of various coverings attached directly to the wheel; and the use of deeper, stiffer, and heavier aerodynamic rims. As winds, and particularly headwinds, are a principal source of energy loss in bicycle locomotion, expensive aerodynamic wheel designs have become increasingly popular. However, these aerodynamic wheel designs have often been tuned to reduce form drag, rather than frictional drag. As a result, augmented frictional drag forces present on these larger-surfaced aerodynamic wheel designs tend to offset much of the gains from reduced form drag forces, thereby negating potential reductions in overall drag.
Bladed spokes, tapered in the direction of motion through the wind, are designed to reduce form drag. These streamlined spokes suffer from increased design complexity, increased weight and higher costs. In addition, such bladed designs are more susceptible to crosswind drag effects: The increased surface area of the bladed spoke can rapidly increase form drag in the presence of any crosswind; any crosswind directed upon the flat portion of the spoke quickly increases pressure drag upon the spoke.
Under low crosswinds, the bladed spoke presents a relatively small forward profile facing oncoming headwinds, minimizing form drag. Indeed, the thin profile of the blade generally minimizes form drag over that of round spoke profile. However, most external winds will not be precisely aligned co-directional with the forward motion of the wheel. Such winds cause a crosswind component to be exerted upon the wheel, leading to flow-separation—and thus turbulence—behind the bladed spoke, and thereby generally negate the potential aerodynamic benefit of the bladed-spoke design. Under high crosswinds, the round spoke profile may even outperform the bladed spoke in terms of drag reduction. Perhaps a result of these conflicting factors, the bladed spoke has not become the common standard for use in all bicycle competitions.
Wheel covers generally include a smooth covering material attached directly to the wheel over the outside of the spokes, generally covering a large portion of the wheel assembly, often extending from the wheel rim to the axle. Wheel covers add weight to the wheel assembly and can result in more wheel surface area being exposed to winds. The additional weight on the wheel is detrimental to wheel acceleration, while the large surface area of the cover can increase frictional drag. Although covering the wheel spokes can reduce form drag forces thereon, the increased frictional drag forces on the larger surface areas can largely offset any aerodynamic benefit. In addition, covering large portions of the wheel also increases bicycle susceptibility to crosswind forces, destabilizing the rider. For this reason, wheel covers are generally used only on the rear wheel of a bicycle, and generally only under low crosswind conditions. Perhaps as a result of these conflicting factors, wheel covers have not become the standard equipment for use in all bicycle competitions.
Recently developed for use on bicycles, deeper, stiffer and heavier aerodynamic wheel rims suffer several drawbacks: deeper (wider along the radial direction of the wheel) and streamlined rims are often used to reduce profile drag on high-performance bicycle wheels. As mentioned, these rims are generally designed to reduce profile drag under various crosswind conditions. However, these deeper rims—having generally larger rotating surface areas—can dramatically increase friction drag. As will be shown, friction drag is particularly increased on the expanded upper wheel surfaces, largely negating any potential benefit of the reduced profile drag. In addition, such deep wheel rims with minimal spokes must be made stronger and stiffer—typically with double-wall construction—than conventional single-wall, thin-rim designs. As a result, such deep rims often ride more harshly over bumpy terrain, and are generally heavier, adding weight to the bicycle, which becomes a drawback when the grade becomes even slightly uphill.
As a result of these and other countervailing factors, no single wheel design has emerged as the preferred choice for reducing drag on bicycle wheels over a wide range of operating conditions. Instead, a variety of wheel designs are often employed in modern racing bicycles. In the same competition, for example, some riders may choose to use bladed spokes, while others choose round spokes, while still others choose deep rims or wheel covers. The differences in performance between these various wheel designs appear to only marginal affect the outcome of most races.
Spoke art includes many examples having rectangular or otherwise non-aerodynamic cross-sectional profiles of wheel spokes for use in automotive applications. Examples include patents U.S. D460,942, U.S. D451,877, U.S. D673,494, U.S. D396,441 and others.
Cycle spoke art includes a tapered spoke of U.S. Pat. No. 5,779,323 where the cross-sectional profile of the spoke changes from more highly elliptical near the wheel hub to more generally oval near the wheel rim. As will be shown, the spoke shown is tapered to minimize—rather than maximize—any aerodynamic benefit, especially when used in the presence of crosswinds.